1. Field of the Invention
The present invention relates to a process for the fast doping of semiconductors.
2. Description of the Prior Art
It is known that doping of a semiconductor consists in implanting foreign particles, or impurities in a substrate, constituted by the semiconductor to be doped, and in rendering these impurities electrically active. The addition of impurities in a semiconductor enables the physical properties of said semiconductor to be modified. This semiconductor may be in the form of a single crystal and/or may already be doped.
The main applications of doped semiconductors are the manufacture of different components of the diode, transistor, thyristor, etc . . . type and the manufacture of solar cells.
Several processes for doping semiconductors are known, such as for example, ion implantation, diffusion, epitaxial growth, alloying technique, etc . . . . The different processes of doping used depend on the impurities with which the semiconductor is doped, on the material constituting the latter and on the type of doping which it is desired to obtain.
In these different processes of doping, the addition of the impurities in the substrate is a slow process which generally lasts several hours, or at least several minutes. This is so, particularly in the case of the process of doping by ion implantation. This process of doping consists of two distinct steps:
The first step consists in implanting the substance by means of an ion or particle beam, the section of which is of the order of a cm.sup.2 or mm.sup.2. The point of impact of the particle or ion beam on the substrate moves so as to cover the whole surface of the substrate in homogeneous manner. This first step, or pre-deposit, it generally slow.
Studies have shown that two ions or particles never arrive at the same time on the substrate. Moreover, the variations between the instants of arrival between two particles cause the energy to dissipate progressively in the whole mass of the substrate. The heating produced when the particles penetrate in the substrate remains very little and cannot produce the effect of annealing necessary for rendering the impurities electrically active and to allow the crystal lattice which was disturbed during bombardment of the substrate by the particles or ions, to be rearranged. This annealing step constitutes the second step of this doping process. This annealing consists, for example, in taking the sample of the implanted substrate to different temperatures (between 0.degree. and 1400.degree. C.) for a variable length of time, which may be as much as several hours, in suitable furnaces.
For this second step, a so-called transitory annealing is also used, which consists in taking an energy density to the surface or to the level of the first implanted layers, for a very short time, so that very high temperatures are reached locally during this time. Studies have shown that the best results are generally obtained when the energy supply is sufficient to liquefy the first layers of the substrate.
Energy may be supplied in different manners and in particular with the aid of a light energy emitter means, for example a laser or a light flash, or pulsed electron beams.
This method of doping, by ion implantation, presents the drawback of consisting of two steps and, like all the other methods mentioned hereinabove, the different steps of the process are long.
It is therefore an object of the present invention to provide a process for the fast doping of semiconductors which enables these drawbacks to be overcome and, in particular, avoids the annealing phase by reducing the time for implantation of the particles or ions in the substrate.